Astronomers searching for forming planets have a new place to look. Even the thin disks around brown dwarfs are capable of forming grains large enough that, one day, they could potentially coalesce into a rocky planet.Planets — including Earth — are the leftovers of star formation. As a proto-stellar cloud collapses, most of the gas and dust folds into the infant star. What’s left distributes into a disk and continues to coalesce, forming asteroids comets, and planets.
While debris disks have been observed around many young stars, the exact details of how planets form in these disks is an open question. Now, with new observations from one of the most sensitive telescopes on the planet, astronomers have added yet another clue to the planet-formation mystery.
Astronomer Luca Ricci (California Institute of Technology) led an international team that recently released results stemming from observations of the brown dwarf Rho-Oph 102. They observed the failed star using the combined power of 15 to 16 radio antennas, part of the Atacama Large Millimeter/submillimeter Array (ALMA). Located high in the Chilean desert, ALMA’s dishes will number 66 when the telescope finishes construction in 2013. But already the collection of 7-meter and 12-meter antennas makes up one of the most sensitive telescopes in the world. ALMA has been conducting science observations since the end of 2011.The brown dwarf Rho-Oph 102, named for the star-forming region it calls home, Rho Ophiuchi, was one of the first objects chosen to be observed by ALMA because it showed signs of harboring a thin disk of dust and gas. Brown dwarfs are far too massive — in this case 60 times the mass of Jupiter — to be called planets, yet unlike their heavier stellar cousins, they lack the mass needed to sustain fusion in their cores. Rho-Oph 102’s small mass led astronomers to believe its debris disk would be similarly puny, too sparse to ever form planets.
The new ALMA observations dispute this idea, suggesting instead that planet formation around brown dwarfs might even be common. Ricci and his team observed the disk of the brown dwarf at two different wavelengths and compared the relative brightness as a way of measuring the size of the particles in the cold, outer part of the disk. To Ricci’s surprise, the particles around Rho-Oph 102 were millimeter-sized, much larger than expected. Though they’re tiny now, these grains are the building blocks that could eventually form rocky planets.
In other words, even the smallest stars and brown dwarfs might be capable of hosting rocky planets like Earth. Since low-mass stars are the most common type of star, rocky planets may be even more common than previously thought. And figuring out just how all those planets formed will definitely keep astronomers on this planet busy for quite some time.